One example of typical wet-type multiplate clutch constructions is illustrated in FIG. 1. A multiplate clutch 10 is equipped with separator plates 3 and friction plates 4 as frictionally engaging elements to transmit rotation between a clutch case 1 and a hub (not shown) between which there is relative rotation. In the illustrated example, the separator plates 3 are maintained at outer peripheries thereof in fitting engagement with the clutch case through splines, while the friction plates are maintained at inner peripheries thereof in fitting engagement with the hub (not shown) through splines.
Designated at numeral 2 is a piston for pressing the frictionally engaging elements to bring the clutch into engagement, and by charging pressure oil into an oil compartment 7 through an oil hole 71, the piston 2 is pressed rightwards. When the pressure oil is discharged from the oil compartment 7, the piston 2 is caused to return leftwards by a return spring 6 interposed between the piston 2 and a piston support 5 so that the binding of the frictionally engaging elements is cancelled. At this time, oil enters between the piston 2 and the piston support 5 through an oil hole 61 under centrifugal force, and acts to press the piston 2 leftwards. FIG. 1 also shows splines 8 through which the clutch case 1 is maintained in fitting engagement with a center shaft (not shown), a stopper ring 9 for preventing any rightward movement of the frictionally engaging elements, and a stopper ring 51 for the piston support 5. An alternating long and short dash line X—X indicates a central axis.
Friction plates and separator plates, as frictionally engaging elements in a multiplate clutch, may be provided with waves to avoid mutual sticking and hence, to reduce a drag torque which is produced upon disengagement of the clutch. When the piston 2 is provided with pressing raised portions 21 as illustrated in FIG. 2, the positions of the pressing raised portions 2 of the piston 2 may, however, happen to register with the positions of swell portions or recess portions of the waves on the frictionally engaging elements as shown in FIG. 5. When such positional registration takes place, the distribution of a bearing stress under a load from the piston 2 becomes uneven. Such an uneven distribution of a bearing stress leads to occurrence of various inconveniences, such as a reduction in the coefficient of friction, production of vibrations and noises and a reduction in torque capacity, upon initiation of frictional engagement.